Most fuels, e.g. fuel oils or coals or gaseous fuels (e.g. natural gas or gases produced in gasifiers) generally contain sulfur and/or sulfur compounds hereinafter referred to generally in this specification and appendant claims as sulfur, and also other undesirable substances such as, e.g. vanadium and sodium which give rise to corrosion problems and also pollute the atmosphere. To deal with these problems, several approaches have been proposed, namely:
(1) purifying the fuels before use to remove the undesirable substances; PA1 (2) purifying the fuels during combustion; PA1 (3) purifying the gaseous products of the fuel combustion; and PA1 (4) combinations of (1) to (3).
Proposal (1) has the drawback of being expensive in practice, as also does proposal (3).
Proposal (2) is less expensive than proposals (1) and (3) in principle, but in practice, the operating cost and fuel consumption are relatively high and susceptible of considerable improvement. U.S. patent specification 3,402,998 of A. M. Squires exemplifies one proposal for purifying fuel during part-combustion and for purifying the resulting gases (i.e. proposals (2) and (3)) but wherein the fuel consumption is excessive for the fuel which is part-combusted as a result of the operating cycle. In U.S. Pat. No. 3,402,998, a sulfur-containing fuel oil is partially combusted within the dense phase of a fluidized bed of particles comprising calcined dolomite at temperatures below the range of temperatures at which chemical deactivation of the calcined dolomite occurs and at a pressure of between 4 and 100 atmospheres, and preferably at about 550 psia. The bed is fluidized by passing an oxygen-containing gas into the bottom thereof and sulfur from the fuel is fixed by chemical combination with the calcined dolomite to form sulfurized dolomite (MgO.CaS), the fuel being converted to a substantially sulfur-free combustible gas at about 550 psia. The rate at which the oxygen-containing gas is passed into the bottom of the fluidized bed is sufficient to entrain out of the bed for dilute phase transport sufficient solids to provide adequate time of contact between the fuel and combustible gas products and the sulfur-fixing solids for an acceptable degree of sulfur-fixing in the solids and to maintain the proportion of unreacted calcined dolomite at a sufficiently high level to maintain the sulfur-fixing ability of the fluidized bed. The sulfurizied solids are separated from the substantially sulfur-free combustible gas by a cyclone separator system and treated with CO.sub.2 to convert nonsulfurized CaO to CaCO.sub.3, and with steam to convert CaS to H.sub.2 S by the endothermic reaction: EQU CaS+H.sub.2 O.fwdarw.CaO+H.sub.2 S.
The foregoing treatments are conducted under non-oxidizing conditions, preferably reducing conditions, to avoid converting the CaS to CaSO.sub.4.
The resulting solids mixture of MgO.CaCO.sub.3 and MgO.CaO is returned to the dense phase fluidized bed for further use in fixing sulfur from sulfur-containing fuel. Optionally, the said solids mixture is calcined, preferably under non-oxidizing conditions, to convert to CaCO.sub.3 to CaO before the solids enter the fluidized bed. The process of U.S. Pat. No. 3,402,998 is relatively inefficient for the following reasons:
(a) the amount of oxygen-containing gas passed into the base of fluidized part-combustion bed must be sufficient to entrain solids out of the bed at a rate which depends on the amount of sulfur fixed in the solids from the fuel and the resulting gas products. As a consequence, the amount of heat removed from the bed in the combustible gas containing entrained solids must be balanced by burning in the bed sufficient additional fuel to compensate for the heat loss from the bed.
(b) the amount of solids removed by entrainment from the part-combustion bed is a function of the flow of gases through the bed rather than a function of the amount of sulfur fixed in the bed. As a consequence, the sulfur-fixing solids of the bed are not efficiently utilized.
(c) the reactions for removing sulfur fixed in the bed solids must be performed under suitable conditions for carbonating untreated CaO with CO.sub.2, which means that the pressure must be higher and/or the temperature lower during this operation than the pressure and/or temperature during the sulfur-fixing operation. In practice, the process of U.S. Pat. No. 3,402,998 is operated substantially isobarically and as a consequence, the sulfur-removing reactions must be performed at considerably lower temperatures than the sulfur-fixing reactions. Moreover, the reaction of CaS with steam is endothermic. Thus, a great deal of the heat produced by part-combustion in the fuel must be utilized to raise the temperature of recycled solids, following CO.sub.2 and steam treatment, to the fuel conversion temperature, so that the efficiency of fuel utilization is still further reduced.
Although the process of U.S. Pat. No. 3,402,998 is represented to be, and might appear to be, an integrated process in the sense that the process steps in one part of the process scheme operate in a cooperative manner with process steps in another part of the scheme, in fact the steps concerned with removing fixed sulfur from the sulfurized dolomite operate independently of the steps in which the sulfur of the fuel is chemically fixed in the dolomite solids.
It has been proposed in U.S. patent specification No. 1,244,280 (Basset) to reduce CaSO.sub.4 by passing particulate CaSO.sub.4 material down a rotating kiln having an upstream reducing zone wherein most of the CaSO.sub.4 is progressively reduced to CaS as it passes through the reducing zone, and the resulting solids then pass through an oxidizing zone wherein the following reactions occur progressively: EQU 2CaS+30.sub.2 .fwdarw.2CaO+2SO.sub.2 ( 1) EQU CaS+20.sub.2 .fwdarw.CaCO.sub.4 ( 2) EQU 3CaSO.sub.4 +CaS.fwdarw.4CaO+4SO.sub.2 ( 3)
According to U.S. Pat. No. 1,244,280, the complete decomposition of CaSO.sub.4 is obtained by (reaction with) an excess of CaS (equation (3) above), and the remaining CaS is then oxidized to CaO using an excess of air.
It has been said that the reactions which occur in a fluidized bed reaction system are equivalent to those in a non-fluidized reaction system. This may be the case in some instances, but in the case of the process of U.S. Pat. No. 1,244,280 which takes place in a rotating kiln, the nature and reacting solids and gases in contact therewith varies from position to position in the kiln. Thus the solids entering the oxidizing zone are rich in CaS and at a relatively low temperature, whereas the gases in contact therewith are relatively depleted in oxygen, rich in SO.sub.2 and very hot. The solids approaching the exit of the oxidizing zone are very hot and the gases in that region are rich in oxygen and cool. It has been found that the mol. ratios of oxygen, SO.sub.2 and CaS and the temperature of contact are very influential in determining which of reactions (1), (2) and (3) predominates. When the temperature is above 1050.degree. C. at atmospheric pressure, a low mol. ratio of O.sub.2 to CaS causes reaction (1) to predominate provided that the partial pressure of SO.sub.2 is not so high that reaction is not possible. No temperatures are given in U.S. Pat. No. 1,244,280 but it seems likely that at the entrance to the oxidizing zone, the solids temperature will be too low for much reaction to occur.
As the solids progress down the kiln, they will heat up and pass through a region where the mol. ratios of O.sub.2, SO.sub.2 and CaS and the temperatures thereof will be correct for reaction (1) to predominate. Reaction (1) is highly exothermic, and the solids passing towards the exit will become progressively hotter while the O.sub.2 to CaS ratio will increase, and the air temperature diminish. It is known that at high O.sub.2 to CaS ratios below 1050.degree. C. reaction (2) predominates. Now although reaction (2) is even more highly exothermic than reaction (1), the amount of heat-producing solid will be diminished towards the exit of the oxidizing zone and the amount of air capable of removing the heat will tend to be relatively so great that a significant proportion of the CaS will be converted to CaSO.sub.4 in the product. Moreover, the product solids will be cooled virtually to ambient air temperatures. Thus, the process of U.S. Pat. No. 1,244,280, particularly where it involves the countercurrent contacting of cool air and hot CaS material, produces a hot gas removing virtually all the heat content of the solids and containing SO.sub.2 and unused O.sub.2, and a cool solid containing CaO and CaSO.sub.4.
Theoretically one might substitute the oxidizing part of the rotary kiln of U.S. Pat. No. 1,244,280 for the CO.sub.2 and steam treating steps of U.S. Pat. No. 3,402,998 as has been suggested at a date later than the claimed priority date of the present patent application, but the thermal efficiency of the thus theoretically modified process of U.S. Pat. No. 3,402,998 would be very low because the solids leaving the kiln and entering the fluidized fuel conversion bed would require heating to the temperature of the latter and additional combustion of fuel would be required to provide the heat to raise the temperature of the solids. Alternatively, it is possible, in theory, that the solids could have a higher concentration of unconverted CaS at a temperature approximating to that of the fluidized fuel conversion bed. However, this would have the drawback of circulating a relatively large inventory of CaS from the fuel conversion bed to the rotary kiln and then back to the bed. Such circulation is undesirable in principle, and would, in practice, reduce the efficiency of fixing the sulfur from the fuel in the bed solids and necessitate the use of increased quantities of gas to effect the dilute phase transfer of the solids from the fluidized fuel conversion bed to the top end of the rotary kiln. All of the foregoing ignores the considerable practical difficulties of constructing a rotary kiln to operate at the high pressures of the process of U.S. Pat. No. 3,402,998.